In space conditioning systems, such as air conditioners, heat pumps and refrigeration systems, wherein a phase change refrigerant is used as the heat transfer medium, two heat exchangers are typically used, one to substantially evaporate liquid refrigerant to cool an external fluid such as air passing through the evaporator, and the other as a condenser to substantially condense vapor refrigerant by transferring heat to an external fluid passing through the condenser.
Heat exchangers having refrigerant conduits of relatively flat cross-section are known in the art. Such heat exchangers are often referred to as "parallel flow" heat exchangers. In such parallel flow heat exchangers, the interior of each conduit is divided into a plurality of hydraulically parallel flow paths of relatively small hydraulic diameter (e.g., 0.070 inch or less), which are often referred to as "microchannels", to accommodate the flow of heat transfer fluid (e.g., a phase change refrigerant) therethrough. Parallel flow heat exchangers may be of the "tube and fin" type in which tubular conduits are laced through a plurality of heat transfer enhancing fins or of the "serpentine" type in which serpentine fins are coupled between the conduits. The relatively small hydraulic diameter flow paths enhance heat transfer between a fluid such as a phase change refrigerant flowing inside the heat exchanger conduits and an external fluid such as air flowing through the heat exchanger on the outside of the conduits, particularly when the heat exchanger is used as a condenser.
However, when parallel flow heat exchangers are used as evaporators, performance is degraded by the uneven distribution of liquid refrigerant in the various flow paths. This uneven distribution results in some flow paths having too much liquid refrigerant and some having not enough. One approach to solving the aforementioned problem of uneven refrigerant distribution in an evaporator is described in U.S. Pat. No. Re. 35,502. This patent shows an evaporator having an inlet header with two inlets at respective opposed ends thereof to generate streams of incoming liquid refrigerant, which impinge upon one another to dissipate the kinetic energy and/or momentum of the streams, and an outlet header with two outlets at respective opposed ends thereof to generate two streams of outgoing vapor refrigerant, which reduces outlet resistance. The configuration of the inlet and outlet headers results in a more uniform flow of the refrigerant through the evaporator flow paths. Although some improvement in refrigerant distribution is achieved using this approach, uneven distribution of refrigerant still results because of the mixed phase (i.e., liquid and vapor) refrigerant entering the evaporator.
There is, therefore, a need for improved refrigerant distribution among the flow paths of an evaporator and in particular among the flow paths of a "parallel flow" evaporator.